Wind profiler systems are sophisticated radar systems designed to measure the doppler shift in the very weak echos scattered by inhomogeneities in the index of refraction of the atmosphere above the radar station. These systems are intended to allow continuous observation of the wind above a station. Presently, these units have a high cost, typically between three and four hundred thousand dollars a station. The antenna alone accounts for about a third of the cost. The antennas presently used are typically composed of two large, separate arrays of coaxial-colinear (COCO) strings which are positioned orthogonal to each other to provide two beams which generate information on the east-west and north-south components of the wind above the station, and also to provide a third beam to allow measurement of any vertical component of the wind. Thus, the typical wind profiler antennas have three beams which are switched sequentially to a north, east and vertical position. The north and east beams are usually 15.degree. from the zenith, an angle which is a compromise between the strength of the return/echo, the magnitude of the radial wind horizontal component, and the beam separation at maximum altitude. The last factor is important when the wind field is not uniform.
The conventional COCO wind profiler antenna is a string of half-wavelength sections of coaxial cable, with the inner conductor of each section being connected to the outer conductor of the next and the outer conductor of each section connected to the inner conductor of the next. Thus, the outer conductor has the same phase over the length of the string and the radio wave propagates in a direction normal to the string. A wind profiler requires a tilted beam, normally about 15.degree. from the zenith, so that the horizontal component of the wind can be extracted from the radial velocity which causes the doppler shift in the return echo. The beam is tilted in such an antenna by shifting the phase appropriately between the numerous strings which make up the antenna array. Thus, one array can only have the beam radiated from it tilted in one direction. Consequently, the conventional COCO wind profiler antenna requires two orthogonal arrays to provide the north-south as well as the east-west components of the winds. The cable required to feed the antenna strings with signals of the proper phase is very complex. Typically, there is more cable in the feed system than in the antenna itself. The two orthogonal sets of coaxial-colinear strings are mounted above a ground reflecting plane with each string about a half wavelength apart to form a mesh which is typically about 22 wavelengths on a side.
The basic coaxial-colinear or COCO antenna has been used in a variety of radar and communications antenna systems for many years. An early description of this antenna system is provided in the U.S. Pat. No. 2,158,376 to Moser, et al. A complete review of the COCO antenna is provided in a Ph.D. thesis by Thierry J. Judasz at the University of Colorado, August 1987, entitled "The Coaxial Colinear Antenna: Theory and Experimental Models."
Normally, the cable element length L of a COCO antenna is chosen so that L=.lambda..sub.c /2, where .lambda..sub.c is the wavelength in the coaxial cable. When these conditions are met, a phase reversal occurs at each section joint and a nearly uniform phase exists along the entire string. Under such conditions, the E field propagates in a direction normal to the string. This antenna can also be considered resonant since the energy reflected from the end of the string can be made to reinforce the primary wave. Special treatment of the end of the array with a quarter wavelength shorted stub and another quarter wavelength extension increases the reflection efficiency and enhances the performance of the array. If, however, the transmitter frequency changes from the frequency for which the antenna was designed, the beam is tilted from the normal position and reflections from the end of the array generate a second beam at a negative angle relative to the first one, with the second beam being weaker than the primary beam. The exact angle of the beams depends on the phase relationships following the reflection. It is possible to terminate the end of the array to remove the reflection with an appropriate resistor, but to do so produces an undesirable loss of power.